Apparatus for continuous photoelectrophoretic imaging



Feb. 1l, 1969 v. s. MIHAJLov APPARATUS FOR CONTINUOUS PHOTOELECTROPHORETIC IMAGING Filed April 18, 1966 UWENTOR.l VSEVOLOD S. MIHAJLOV BY A frog/v5.1

United States Patent 3,427,242 APPARATUS FOR CONTINUOUS PHOTO- ELECTROPHORETIC IMAGING Vsevolod S. Mihajlov, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 18, 1966, Ser. No. 543,125 U.S. Cl. 204--300 Int. Cl. B01k 5/00 11 Claims made through the lbottom of this plate while a second cylindrical shaped electrode is rolled over the top of the suspension, The particles are believed to bear an initial charge when suspended in the liquid which causes them to 4be attracted to the transparent lbase electrode and to change polarity by exchanging charge with the base electrode upon exposure so that exposed particles migrate across to the roller electrode to form an image on the base electrode by particle subtraction. The' system may be used to produce monochromatic images by using a single color particle in the suspension or a number of differently colored patricles in the suspension which all respond to the same wavelengths of light exposure. In polychromatic systems, mixtures of two or more differently colored particles, each of which are sensitive only to light of specific, different and generally narrow wavelengths are used. Thus, for example, a `full color image may be produced -by using a mixture of cyan, magenta and yellow particles which respond to red, green, and blue light, respectively. An extensive and detailed description of the photoelectrophoretic imaging technique as described above is found in copending U.S. applications Ser. Nos. 384,737 now U.S. Patent 3,384,565; 384,681; now abandoned and 384,680 now abandoned, led Jan. 23, 1964.

Although it has been found that good quality images c-an be produced, especially when a relatively insulating blocking electrode surface is used on the roller, the photoelectrophoretic imaging process is relatively slow, generally utilizing reciprocating ymovement as the imaging technique. A continuous imaging system based on particle migration techniques has been suggested in the prior art as described, vfor example, in U.S. Patent No. 2,940,847. However, this syestem has proven to be so light insensitive, producing such poor images, and is so co-mplex and difficult to prepare that it has never been accepted commercially. This continuous prior art system employs complex parti-cles including at least two, and frequently more, layers of various different materials including, for example. photoconductive cores with varying high resistivity light filtering overcoatings and sometimes includes glass cores, encapsulated dyes and similar complicating components, which were previously thought to be necessary to provide light filtering action to prevent particle interaction and oscillation in the system, and perform other functions. Furthermore, the exposure station and image source is detrimentally located within the imaging drum. It has, however, been discovered quite unexpectedly and surprisingly in accordance with the present invention that this complex system is not only unnecessary but also undesirable and that instead the continuous system of the present invention may incorporate single component particles made up of colored photosensitive pigments in a conveniently operable arrangement to produce excellent results as described hereinafter.

It is, therefore, an object of this invention to provide a continuous imaging system which will overcome the above-noted disadvantages.

It is a `further object of this invention to provide a novel continuous imaging system capable of direct positive imaging.

Another object of this invention is to provide a continuous imaging system for producing images in one or more colors.

Still a further object of this invention is to provide a novel continuous photoelectrophoretic apparatus.

The foregoing objects and others are accomplished in accordance with this invention, generally speaking, by providing an apparatus whereby an image is internally projected upon the surface of an imaging roller, the imaging roller contacting at the position of image reflection a development roller which has coated on the surface thereof photosensitive pigment particles dispersed in an insul-ating liquid carrier. By the process of photoelectrophoresis, as described above, a direct positive image is formed on the surface of the imaging roller. The imaging roller is referred to as the injecting electrode and consists of a transparent electrode which may, `for example, be made up of a layer of optically transparent glass overcoated with a thin optically transparent layer of conductive material, such as tin oxide. The development roller is made up of a central core which preferably has a fairly high electrical conductivity and is usually covered with a layer of blocking electrode material, such as Baryta paper. It is referred to as the blocking electrode. The positive, developed image is then contacted with a transfer `substrate, such as paper, for the purpose of transferring the developed image from the imaging roller onto the surface of the substrate. The transferred pigment image is then fixed in a suitable manner to the surface of the transfer sheet.

The invention is illustrated in the accompanying dra-wings in which there is shown a side sectional view of an exemplary continuous photoelectrophoretic printing apparatus of the present invention.

Referring now to the drawing, there is seen a rotary transparent electrode 1 in the form of a drum which, in this instance, is made up of a layer of optically transparent glass 2, carrying on its outer surface a thin optically transparent conductive layer 3 of tin oxide. Tin oxide coated glass of this nature is commercially available under the trade name NISSA` glass. This rotary base electrode is referred to as the injecting electrode. In close proximity to electrode 1 and parallel thereto is a rotary developing electrode 10 made up of a central core 11 which is preferably of fairly high electrical con-ductivty and is covered with a layer of blocking electrode material 12 which may, for example, be Baryta paper. Although a blocking electrode material such as 12 need not necessarily be used on the surface of electrode 10, the use of such a layer is preferred because of the markedly imporved results which it is capable of producing. For obvious reasons the latter electrode, as stated above, is referred to as the blocking electrode. A detailed description of these improved results and the types o-f materials which may be employed as the electrode coating material is described in detail in copending U.S. application Serial No. 384,689 referred to above. During the imaging process a thin suspension layer 14 of finely divided photosensitive particles dispersed in an insulating carrier liquid is formed on the surface of the developing or blocking electrode 10 supplied from reservoir 15. Within the rotary imaging roller (electrode) 1 is located a stationary, first-surface mirror which receives the image projected from the transparent subject to be copied 2S and directs the reected image through the exposure slit 22 and onto the surface of the suspension at the line of contact 21 where the injecting electrode 1 contacts the developer coated blocking electrode 10.

The transparency 25 that is to be printed is shown passing under the light source 26 and moving synchronously with the rotary imaging drum 1. From the image `formation station or lens -27 by way of the imaging slit 17, the image is projected onto the surface of the first surface mirror 20` which is located within the imaging drum and, in turn, reflected onto the surface of the suspension at the line of contact 21. The image is preferably projected in a plane normal to the surface of the drum so as to minimize distortion of the image and loss of image density. The imaging injecting electrode drum is grounded by connection 30 and the developing blocking electrode is connected with a potential source 31 which is in turn connected to ground. The polarity of the charge on the developing electrode 10` is opposite to that of the polarity that is carried on the surface of the imaging electrode 1.

The image projector made up of light source 26, double lens complex 28, transparency 2S and a single lens 27 is provided to expose the suspension 14 made available from reservoir 15 to the light image of the original transparency to be reproduced. During imaging, drive means 52 rotates the development electrode 10, synchronously with the imaging electrode 1, thereby carrying the imaging suspension containing the photosensitive radiant energy particles into intimate contact with the surface of the injecting electrode at the site of projection of the image from transparency 25 onto the surface of the suspension at the line of contact 21. This slit exposurecauses selective particles to migrate through the dispersion liquid and adhere to the surface of the rotating blocking electrode 10` leaving behind a positive particle image on the surface of the injecting electrode 1 which is a duplicate of the original transparency 25.

The image now developed on the surface of the imaging (injecting) electrode is carried so as to contact an adhesive copy web 41 which is passed up against the drum sur-face by two idler rollers 42 and 45 so that the web moves at the same speed as the periphery of the drum. A transfer roller 43 is placed behind the web 41 and between rollers 42 and 45. Thus, the developed image is adhesively transferred from the surface of the imaging or injecting electrode onto the surface of the copy web. The copy web may consist of adhesive tape which will pick up the complete image as it passes over the transfer roller. The transferred image may then be fixed in place as, for example, by placing a lamination over its top surface or by virtue of the presence of a dissolved binder material in the carrier liquid such as a paraffin wax or other suitable binder that comes out of solution as the carrier liquid evaporates. In fact, the carrier liquid itself may be a molten paraffin wax or other suitable lbinder in a liquid state which is selffixing upon cooling and return to the solid state. The fixing step is represented by fixing unit 44. The remnants of the photosensitive dispersion are removed from the surface of the developing electrode 10 by brush 50 and from the surface of the imaging electrode by brush 51.

It is to 4be understood that during the imaging process of the present invention as a result of the nature of photoelectrophoretic imaging when a positive image is formed on the surface of the injecting electrode a corresponding negative image is formed on the surface of the blocking electrode. When the system is a monochromatic one it may be desirable to transfer both the positive and negative developed images to respective transfer sheets. Therefore, if desired a transfer system similar to that described above in conjunction with the injecting electrode transfer may also be included in the apparatus to enable the transfer of the image formed on the blocking electrode. Alternatively, when used, the blocking electrode layer of material, such as Baryta paper, may be removed from the development roller to preserve the negative image recorded thereon. A conductive imaging substrate, such as a conductive cellophane web, may be driven between the blocking and imaging electrodes so as to permit the formation of the positive image on the surface of the imaging substrate thereby eliminating the transfer step required by the embodiment disclosed in the illustration and further eliminating the need for cleaning the surface of the imaging or injecting electrode. Any suitable transparent conductive material may be used as the imaging substrate. Typical optically transparent conductive materials are cellulose acetate (optical grade), polyesters such as Mylar, a polyethylene terephthalate, and polycarbonates, such as Plestar, overcoated by any suitable means such as by vacuum deposition with a transparent metal coating such as aluminum, copper, gold, silver or chro-mium.

Materials which may be used as the photosensitive particles and carrier liquid, the injecting electrode and blocking electrode, are to be found in the copending U.S. applications, Ser. Nos. 384,737, 384,680, and 384,681, mentioned above and copendin-g U.S. applications, Ser. Nos. 468,935 and 473,607, all of which are accordingly incorporated herein by reference. Processing parameters such as the Amagnitude of the applied potential and the like may also be found in these copending applications. The present system has been found to be capable of producing either monochromatic or polychromatic images depending upon the color, sensitivity and number of different pigments suspended in the carrier liquid and the color light to which the suspension is exposed. Further-more, it is to be understood that it is not intended that the structural arrangement of the apparatus described in the present invention be restricted to the design as set-out herein, and it is intended to include all similar configurations which will satisfy the requirements of the present invention. For example, it is not necessarily required that the blocking and injecting electrodes be of the same dimension as they appear in the accompanying illustration. Nor is it necessary to limit the number of development electrodes in the system, and when multiple development is desired two or more electrodes may be incorporated in the apparatus.

The term injecting electrode when used in the course of this invention should be understood to mean that it is an electrode which will preferably be capable of injecting electrons into the bound photosensitive particles of the dispersion when it is exposed to light so as to allow for a net change in the charge polarity on the particle. By the term blocking electrode is meant one which is incapable of injecting electrons into or receiving electrons from the above-mentioned photosensitive particles at more than a very slow rate when the particles come into contact with the surface of the electrode. Obviously, if all polarities in the system are reversed the function of the electrodes will also be reversed.

It is preferred that the injecting electrode be composed of an optically transparent glass drum overcoated with conventional conductive materials such as tin oxide, copper, copper iodide, gold or the like to obtain optimum results; however, other suitable materials including many semi-conductive materials such as raw cellophane which are ordinarily not thought of as conductors but which are still capable of accepting injected charge carriers of` the proper polarity under the infiuence of the applied field may be used in the course of this invention. Even highly insulating materials such as polytetrafluoroethylene may be placed over the surface of the injecting electrode and such electrodes still may be operative because charge, which leaves the photosensitive particles initially bonded on this surface, upon exposure to light, can merely move out of the particles and remain on the insulating surface thereby allowing the exposed particles to migrate. However, as mentioned above, the use of the more conductive materials is preferred because it allows for cleaner char-ge separation and the charge leaving the particles upon exposure can -rnove into the underlying surface and away from the particles in which it originated. This also prevents possible charge buildup on the electrode which might tend to diminish the inter-electrode field. The blocking electrode, on the other hand, is selected so as to prevent or greatly retard the injection of electrons into the photosensitive particles when the particles reach the surface of this electrode. The blocking electrode drum generally will consist of a very soft central core which may be of fairly high electrical conductivity. Typical conductive materials are conductive rubber, and metal foils of steel, aluminum, copper, and brass. Preferably, the central core Iwill have a high electrical conductivity in order to establish the required polarity differential. However, if a low conductivity core is used, a separate electrical connection may be made to the back of the blocking layer of the electrode when the latter is utilized. It is preferred that the blockin-g layer when used be either an insulator or a semi-conductor which will not allow for the passage of sufiicient charge carriers under the influence of the .applied field to discharge the particles finally bound to it thereby preventing particle oscillation in the system which results in enhanced image density and image resolution. Even if this blocking electrode will allow for the passa-ge of'some charge carriers through it to the photosensitive particles, it will still be considered to come within the class of preferred materials if it does not allow for the passage of sufficient carriers to recharge the particle to the opposite polarity because even a discharge particle will tend to adhere to this blocking electrode by Van Der Waals forces. Here again, other suitable materials not coming within the preferred class may be used, as defined in copending ^U.S. application Ser. No. 384,680. Exemplary of t-he preferred blocking material is Baryta paper Iwhich consists of a paper coated with barium sulfate suspended in a gelatin solution. The Baryta paper and the other suitable materials used as the blocking layer may be wetted on their back surfaces with tap ywater or coated on these back surfaces with electrically conductive materials. The blocking electrode layer, when used, may be a separate replaceable layer which is either taped on the developing electrode roller or held by mechanical fasteners or any other device which is capable of releasably holding the layer on the roller. In the alternative, the layer may be an integral part of the roller itself, being either Iadhesively bonded, laminated, spray coated or otherwise applied to the surface of the roller.

Any suitable voltage may be applied, depending upon the thickness of the materials used, to produce a preferred field strength of from about 400 to about 1,000 volts per mil across the suspension at the time of exposure in order to obtain optimum results. For example, using a seven mil thick plastic coated Baryta paper over a conductive Neoprene (polychloroprene) backing as the blocking electrode, when this electrode contacts the injecting electrode in the presence of the imaging suspension, it has generally been found that voltages of from about 1,000 to about 7,000 may be employed and that ordinarily better images are produced if the applied voltage is in the higher end of this range. Different voltages may be used with different components of varying thickness to attain approximately the same field strength in the system.

Although this invention has been described for the most part in connection wit-h a Baryta paper blocking electrode, any suitable material havin-g a resistivity of about 107 ohms/ cm.2 or greater may be employed. Typical materials in this resistivity range -which may be employed in the system of the present invention include cellulose acetatel and polyethylene coated papers, cellophane, nitro cellulose, polystyrene, polytetrafiuoroethylene, polyvinyl fluoride, and polyethylene terephthalate.

yAny suitable means may be used to transfer the developed image from4 the surface of the injecting electrode. The transfer step may be carried out 'by lmeans of an ad- :hesive pick-off technique such as is shown .and described above or the transfer of Ithe image may be carried out in the presence of electrostatic field. In addition, other trans rfer methods may be used su-ch as those -disclosed in copending U.S. application Ser. No. 542,050, having a common assignee.

Any suitable fixing means may be used in Ithe course of the present invention to fix the |transferred image to the surface of la copy web such las by placing a lamination over the top surface of the transferred ima-ge or by fvirtue of the inclusion of 4a suitable binder material in the carrier for the photosensitive particles, such as a wlax. In the latter case, the binder material will solidify upon cooling thereby fixing the developed image to the surface of the copy web. Flor ease of operation it is prefer-red to use the lamination technique.

To Ifurther define the speci-fics of the present invention, the following examples are intended to illustrate and not limit the particulars of the present system. Parts and percentages are by weight unless otherwise indica-ted.

`In Ithe following examples, four different imaging suspensions are employed with ythe Iapparatus of the present invention as shown in the accompanying illustration. The imaging or injecting electrode -is made up cf NESA glass, -as described above, `and the surface of the glass is connected to ground. The injecting electrode roller is yapproximately 7 inches in diameter. IThe development or blocking electrode consists of la hard conductive rubber core of Iapproximately 7 inches in diameter coated with a layer olf water-proof Baryta paper. A negative voltage of 2,500 is -applied across -t-he imaging suspension. The imaging suspension used in these lexamples consists of about 7 parts by weight of ph-otosensi-tive particles in Sohio yOd-orless Solvent 3440, a kerosene fraction. The image source used is ya Kodachrome color transparency.

Example II lA tri-mix is prepared by dispersing equal parts of a cyan pigment, Monolite Fast Blue GS, la mixture of the alpha :and lbeta forms of metal-free phthalocyanine; a magenta pigment, Watchung Red B, 1-(4methyl5 chloroazobenzene-2'sulfonic acid)-2-hydrox-3-naphthoic acid, C11. No. 15865, and a yellow pigment, Algol Yellow GC, 1,2,5,6di (C,Cdip`henyl)-thiazole-anthraquinone, C.I. No. 67,300, in the above-mentioned Schio Odorless Solvent 3440. The tri-mix is imaged, as described above, forming a full color image conforming to the original transparency.

Example 'II 2A tri-mix is prepared by dispering equal parts of a Cyan pigment, Diane Blue, 3,3methyoxy4,4' diphenylbis (1azo-2=hydroxy3"-nap'hthanilide), C.I. No. 211- a magenta pigment, Calcium Lit'hol Red, a calcium lake of 1 (2 azonaphthalene 1 sulfonic acid) 2- naphthol, CJI. No. 15,630, and a yellow pigment, 8, 13- dioxodinapht'ho-1,2-2,3) furan 6 carbox 4" methoxyanilide, disclosed in U.S. Patent application No. 421,377 filed Dec. 28, 1964, in the Sohio Oidorless Solvent 3440 mentioned above. The tri-mix is imaged, as described above, forming a :full color image conforming to the original transparency.

Example 'III Seven parts by weight of the metal-free phtha'locyanine pigment disclosed in Example I is blended with the Sohio odorless Solvent and imaged as described above. However, in this instance, a black-and-white transparency is used. A single color cyan image on a background of white is formed conforming to the original transparency.

'Example IV IThe photo sensitive particles of this example consist entirely of Quindo Magenta RV-6803, a substituted quinacridone. After blending the pigment with the Sohio 7 Solvent 3440, the mixture is imaged as in Example IIFI. A magenta image conforming `to the original transparency is formed.

Although the present examples were specific in terms of conditions and materials used, any of the above-listed typical materi-als may be substituted when suita'ble in the above examples with similar results. In :addition to the steps used t-o carry out lthe process of the present invention, other steps or modifications may be used, if desirable. For example, exposure of the transparent image may take place along the longitudinal axis of the imaging (injecting) elect-rode. lIn addition, other materials may be incorporated in the develop-ing suspension, image pnojector, jinjecting electrode, blocking elec-trode, copy web, or iixing unit, which will enhance, synergize or otherwise desinably affect the properties of the systems `for -their present use. For example, the developer suspension may contain sensitizers land/or binders for the photosensitive particles which are dissolved or suspended in the carrier liquid.

Anyone skilled in the art will have other modifications -occur to him |based on the Iteachin-g of the present invention. These modifications lare intended lto be encompassed Iwithin the scope of this invention.

What is claimed is:

1. An apparatus for photoelectrophoretic imaging comprising in combination a rotatably mounted first electrode, means to apply a film of an imaging suspension to the surface of said first electrode, an optically transparent second electrode rotatably mounted in close proximity and parallel to the axis of said first electrode such that the surface of said second electrode is in area contact with the film on the surface of said first electrode, means to rotate said first and second electrodes, means to project an image onto said suspension at the area of contact of said second electrode with said film coated first electrode, and means to apply an electric field across said suspension at said area contact whereby an image is produced on at least one of said electrodes.

2. The apparatus as described in claim 1 wherein said projecting means comprises an exposing mechanism located external to said second electrode and a refiecting mechanism located internal to said second electrode.

3. The apparatus as described in claim 2 wherein said exposing mechanism is adapted to project an image in a plane perpendicular to the surface of said electrode.

4. An apparatus for continuous photoelectrophoretic imaging comprising in combination a blocking electrode cylindrically shaped and rotatably mounted for continuous imaging, means to apply a film of an imaging suspension to the surface of said blocking electrode, an optically transparent, cylindrically shaped injecting electrode rotatably mounted in close proximity and parallel to the axis of said blocking electrode such that the surface of said injecting electrode is in area contact with the film on the surface of said blocking electrode, means to rotate said blocking and injecting electrodes, means to project an image into said suspension at the area of contact of said injecting electrode with said film coated blocking electrode, and means to apply an electric field across said imaging suspension at said area contact whereby an image is produced on at least one of said electrodes.

5. The apparatus as described in claim 4 further including a means to transfer the image from the surface of at least one of said imaged electrodes to a copy web.

6. The apparatus as described in claim 4 wherein said projecting means comprises an exposing mechanism located external to said injecting electrode and a reflecting mechanism located internal to said injecting electrode.

7. The apparatus as described in claim 6 wherein said exposing mechanism is adapted to project an image in a plane perpendicular to the surface of said injecting electrode.

8. An apparatus for continuous photo-electrophoretic imaging comprising in combination a blocking electrode cylindrically shaped and rotatably mounted for continuous imaging, means to apply a lm of an imaging suspension to the surface of said blocking electrode, an optically transparent, cylindrically shaped injecting electrode rotatably mounted in close proximity and parallel to the axis of said blocking electrode, means to direct a transparent conductive imaging substrate between said blocking electrode and said injecting electrode in such a manner so as to form a single continuous area of contact between said injecting electrode and film coated blocking electrode, means to rotate said blocking and injecting electrodes, means to project an image onto said suspension at said area of contact, and means to apply an electric field across said imaging suspension at said area of contact whereby an image is produced on the surface of said imaging substrate.

9. The apparatus as described in claim 8 wherein said conductive imaging substrate comprises a metallized polyethylene terephthalate film.

10. The apparatus as described in claim 1 wherein said projection means comprises an exposure mechanism located such that exposure takes place along the longitudinal axis of said second electrode.

11. The apparatus as described in claim 4 wherein said projection means comprises an exposure mechanism located such that exposure takes place along the longitudinal axis of said injecting electrode.

References Cited UNITED STATES PATENTS 2,732,775 1/ 1956 Young et al. 96-1 3,043,684 7/1962 Mayer 117-372 X 2,940,847 6/ 1960 Kaprelian 96-1.4 3,043,684 7/1962 Mayer 117-372 X 3,276,896 10/1966 Fisher 118-637 3,281,857 10/ 1966 Kaiser 118-637 3,384,566 5/1968 Clark 204-181 FOREIGN PATENTS 624,328 7/ 1961 Canada.

JOHN H. MACK, Primary Examiner.

E. ZAGARELLA, Assistant Examiner.

U.S. Cl. X.R. 

1. AN APPARATUS FOR PHOTOELECTROPHORETIC IMAGING COMPRISING IN COMBINATION A ROTATABLY MOUNTED FIRST ELECTRODE, MEANS TO APPLY A FILM OF AN IMAGING SUSPENSION TO THE SURFACE OF SAID FIRST ELECTRODE, AN OPTICALLY TRANSPARENT SECOND ELECTRODE ROTATABLY MOUNTED IN CLOSE PROXIMITY AND PARALLEL TO THE AXIS OF SAID FIRST ELECTRODE SUCH THAT THE SURFACE OF SAID SECOND ELECTRODE IS IN AREA CONTACT WITH THE FILM ON THE SURFACE OF SAID FIRST ELECTRODE, 